1. Field of the Invention
This invention relates to methods and apparatuses for liquefying natural gas. In another aspect, the invention concerns an LNG facility employing an enhanced nitrogen removal system.
2. Description of the Related Art
Cryogenic liquefaction is commonly used to convert natural gas into a more convenient form for transportation and/or storage. Because liquefying natural gas greatly reduces its specific volume, large quantities of natural gas can be economically transported and/or stored in liquefied form.
Transporting natural gas in its liquefied form can effectively link a natural gas source with a distant market when the source and market are not connected by a pipeline. This situation commonly arises when the source of natural gas and the market for the natural gas are separated by large bodies of water. In such cases, liquefied natural gas (LNG) can be transported from the source to the market using specially designed ocean-going LNG tankers.
Storing natural gas in its liquefied form can help balance out periodic fluctuations in natural gas supply and demand. In particular, LNG can be “stockpiled” for use when natural gas demand is low and/or supply is high. As a result, future demand peaks can be met with LNG from storage, which can be vaporized as demand requires.
Several methods exist for liquefying natural gas. Some methods produce a pressurized LNG (PLNG) product that is useful, but requires expensive pressure-containing vessels for storage and transportation. Other methods produce an LNG product having a pressure at or near atmospheric pressure. In general, these non-pressurized LNG production methods involve cooling a natural gas stream via indirect heat exchange with one or more refrigerants and then expanding the cooled natural gas stream to near atmospheric pressure. In addition, most LNG facilities employ one or more systems to remove contaminants (e.g., water, acid gases, nitrogen, and ethane and heavier components) from the natural gas stream at different points during the liquefaction process.
Frequently, the natural gas stream introduced into the LNG facility can have a relatively high concentration of nitrogen. High nitrogen concentrations in the natural gas feed stream can present several operational problems as the gas is subjected to liquefaction in an LNG facility. For example, the natural gas can be difficult to condense, thereby increasing the compressor horsepower requirements. Liquefying natural gas having an increased nitrogen concentration can also lead to larger volumes of off-spec LNG and lower quality fuel gas for use within the facility. Problems with high-nitrogen natural gas can be further exacerbated when the LNG facility employs one or more open-loop refrigeration cycles that utilize at least a portion of the natural gas feed stream as a refrigerant.
Although highly desirable and even necessary in some cases, conventional processes of removing nitrogen from the natural gas liquefied in an LNG facility can be expensive. Typical nitrogen removal units (NRUs) process large volumes of methane-containing intermediate process streams having relatively dilute, but nonetheless undesirable, concentrations of nitrogen. Processing these larger volumes of more nitrogen-dilute process streams increases the overall cost of nitrogen removal, in terms of capital, maintenance, and operating costs. In order to minimize costs and maximize profit, a more efficient process for removing nitrogen from an LNG system is desirable.